Cetus a Monster Whale

Transcription

1 deepsky delights Cetus a Monster Whale by Magda Streicher magdalena@mweb.co.za The Cetus constella on is situated in the northern part of sky and ranked as the 4 th largest among the documented 88. Cetus in mythology represents the sea monster sent to kill the princess Andromeda, but legends also point a finger of concern to Draco, another northern hemisphere constella on. The constella on occupies a part of sky that houses mainly galaxies and, sadly, few known clusters or nebulae; but it is famously situated at the south pole of the Milky Way. About 5 o from the Taurus border in the northeastern corner of Cetus, magnitude 2.5 alpha Ce (also known as Menkar) appears to be the watchful eye of the whale. The lovely redorange giant star can be appreciated through binoculars as a double star with a magnitude 5.6 visible partner, but it is not a physical companion of alpha Ce. Image source: Stellarium.org sec on again is somewhat slimmer, a frac on longer with a defined p and a small triangle of magnitudes 12 to 13 stars close by (see sketch). With higher magnifica on the dusty dark lane nearly touches the northern edge of the galaxy. This star city forms a nice triangle a few arcminutes to the south of two similar stars, a slightly yellow magnitude 6.7 and a plain creamwhite magnitude 7.5 which also pairs with a magnitude 10 star to its south. It is a very excep onal star field that rounds off this showpiece galaxy in a very dis nc ve way. The magnitude 4 delta Ce star points the way barely 35 east to NGC 1055, one of the most outstanding objects to be found in this constella on. The galaxy displays a so elongated eastwest oval with the middle part slowly brightening to an intense glow. With averted vision the eastern part displays a fadeout p. The western 224

2 cetus a monster whale Another galaxy, quite different in shape and impression, is the wellknown NGC 1068 (Messier 77), half a degree south from NGC 1055, and the only Messier object in the constella on Cetus. NGC 1068 is a peculiar Seyfert galaxy displaying a misty appearance with an irregular shape and a very bright starlike nucleus. With a mo led brightness of about magnitude 8.2 one gets the impression of a faint globular cluster rather than a galaxy. Higher magnifica on, however, reveals so, barely visible, wisps of nebulosity around the edge. The galaxy is just west of a magnitude 9 foreground star gives the impression of two wide open eyes in the dark of night. Even the stars in the field play the game in pairs, which is quite pleasing. This Seyfert system, which exhibits unusually intense and variable ultraviolet emissions from a ny starlike nucleus is probably the sign of gas spinning into a supermassive black hole. It was also one of the first galaxies found to have a large red shi, thus implying that it was receding rapidly along our line of vision. The spiral structure in NGC 1068 (M77) was first noted by the Earl of Rosse. To the east, forming a triangle with the above two galaxies, is NGC 1072, a very faint northsouth spindle which is a real challenge to search out. An easy way to find objects is to look out for triangles among the stars. NGC 936 is situated west in a long, thin triangle with the stars delta and omicron Ce. The galaxy appears to be just a faint, roundish glow lying south of a group of four stars. 225

3 Higher magnifica on offers no improvement of the view with any sign of the fainter companion galaxies either. The area is packed with galaxies, but if you want to search for them you will need really high magnifica on, a very dark, transparent sky and a lot of pa ence. The collar on the mighty whale s neck indicates the famous magnitude 6.5 red giant star omiron Ce, be er known as Mira, which means the wonderful, a name bestowed on it in 1662 by Johannes Hevelius ( ). The star undergoes actual pulsa ons in size and brightness and varies from as dim as magnitude 9 to as bright as magnitude 3 4 and even as high as magnitude 2. The Greek astronomer Hipparchus became the first person to spot the star s light, but recogni on as a variable star was credited to David Fabricius ( ), who spo ed it rising in 1596 and again in The cycle is now es mated to be 11 months or close to the value of 332 days. In 2007 astronomers imaged Mira s ultraviolet smoke trail of about 2 o long le behind in its years of travel through space. Mira, known as a Miratype star, appeared in Johann Bayer s deepsky delights 17 th century catalogue, where it was assigned the Greek le er omicron. A magnitude 12 companion star can be found in a PA of 90. Another lovely red magnitude 9 star can be seen northeast of Mira, which lends a special effect. However the system was previously iden fied as mul ple in the Hipparcos Input Catalogue. The astronomer William Herschel refers to Mira as a star with a deep garnet colour. Mira s cometlike tail stretches a startling 13 lightyears across the sky (see picture). Mira s tail also tells a tale of its history the material making it up has been slowly blown off over me, with the oldest material at the end of the tail having been released about years ago. Globular clusters are concentrated old stellar groups that can be found in the outer Milky Way disc. Whi ng 1 was found on a survey done by Alan Whi ng (Cerro Tololo InterAmerican Observatory) in 2002 that iden fied it as a compact cluster of blue stars which he believes could possibly be a very young globular cluster due to an abundance of low metals. The object was later classified as an open cluster only Mira appears as a small white dot in the bulbshaped structure at right, and is moving from le to right in this view. The shed material can be seen in light blue. The dots in the picture are stars and distant galaxies. The large blue dot at le is a star that is closer to us than Mira. 226

4 cetus a monster whale Whi ng 1 about 5 billion years old (see picture). This object is situated 4 o west of Mira, but do not expect to make an observa on of this very faint and illusive object. Barely a degree to the south a grouping of seven rela vely bright stars forming a sort of triangle impression poin ng southwest. The name the poin ng seven crossed my mind the moment I laid eyes on this asterism, which is best seen in low magnifica on with a widefield eyepiece. A further two degrees south, more or less in the middle part of the constella on, is NGC 779, a rela vely bright, quite large, thin ray of light in an elongated northsouth direc on, with a bright stellar nucleus. Again you need very dark, transparent skies and rela vely high magnifica on to spot this galaxy, as is the case, sadly, with most of this category objects. Quite outstanding are the three stars eta, iota and beta Ce, which form the tail part of the graceful, mighty whale, which can NGC 246. Picture credit, Dieter Willasch be easily iden fied in the southwestern part of the constella on. Situated in the middle area of the indicated triangle is a very excep onal planetary nebula which truly creates an unusual impression. NGC 246 is well known as one of Cetus s special jewels, and is indeed out of the ordinary. The planetary nebula displays a so, rounddish smoke ring that engulfs five foreground stars, with the hot central star very obvious (see picture). Filters will bring out a kno y structure on the planetary surface, which some mes refer to in the appearance of a human skull. If you want only one object that is worth a visit then you need not look any further. The very faint galaxy NGC 255 is situated only 30 to the north. Another special Cetus galaxy NGC 247 can be found 2 degrees south of the magnitude 2 beta Ce also known by the name Diphda. The spindle in a northsouth direc on is quite outstanding against the background star field with a rela vely bright nucleus (see picture). With careful observa on and higher magnifica on dis nct markings and perhaps a few star points can be spo ed on the dusty surface. 227

5 deepsky delights and really high magnifica on it is possible to spot perhaps a few markings on the surface if you are fortunate to have transparent dark skies. NGC 247 Picture credit, Dieter Willasch NGC 247 belongs to the Sculptor Group of Galaxies and is about 7 million lightyears distant. The galaxy NGC 157 is situated virtually on the western p of the whale s fin just slightly northeast of the star magnitude 3.5 iota Ce. The galaxy lies in a northeast to southwest direc on and displays a quite prominent nucleus covered in haziness. With larger backyard telescopes The CanadaFrance Brown Dwarf Survey (CFBDS) has found the coolest brown dwarf yet, not even 350 o Celsius cool enough to show ammonia in its spectrum (Sky and Telescope, August 2008). This star, known as CFBDS J , has now been put into a new proposed spectral class of Type Y stars and is situated about 40 lightyears away in the Cetus. The special star is situated in the far northwest of the constella on, a degree northeast of the galaxy NGC 307, but spare yourself the effort of looking for it. Why not swim with the monster whale and discover some of the special objects sharing the waves in the seas of Cetus! Object Type RA (J2000.0) Dec Mag. Size NGC 157 NGC 246 NGC 247 CFBDS J NGC 779 Whi ng 1 Star Group Omiron Ce NGC 936 NGC 1055 NGC 1068 (M 77) Planetary Nebula Type Y Brown Dwarf Cluster Asterism Miratype Star 00 h 34 m ʹ x x5.5 * 3.4 x * 5.7 x x x